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May-June 2010

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Connoisseur's Choice: Scorodite Hezhou Prefecture, Guangxi Zhuang Autonomous Region, China

Arsenic, particularly in industrial wastes and in both surface and groundwater, is of critical importance with respect to health and other environmental considerations. It is actually a relatively common and very widespread metal and occurs in large amounts in coal ash and similar energy-related products and in many mineral deposits. It was used throughout the southern United States as a major component of bole weevil poison and later in other types of insecticides and herbicides. Residues resulting from the use of these latter products are the subject of major cleanup efforts today. In recent years, the limit considered acceptable for safe drinking water has been reduced considerably and is now in the low parts per billion, suggesting that even the smallest amounts of naturally occurring, even slightly soluble arsenic minerals, could negatively impact drinking water supplies. Although one must wonder how safe our beautifully colored arsenate specimens really are, most are essentially insoluble in water and considered harmless. One such arsenate, scorodite, has even been used in an attempt to remediate arsenic-bearing groundwater caused by overuse of insecticides and herbicides. In this instance, its formation is induced by injecting iron-rich solutions into shallow wells to sequester biologically available arsenic in the essentially insoluble scorodite thus formed. Consequently, this mineral, which occurs in spectacularly beautiful though quite rare specimens, has been suggested for this issue's Connoisseur's Choice mineral.

Dr. Robert B. Cook, an executive editor of Rocks & Minerals, is a professor emeritus in the Department of Geology and Geography at Auburn University, Auburn, Alabama. He welcomes suggestions for this column.

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